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Ask a Master Gardener: Mulch volcanoes can kill trees

Mulch should not be piled against a tree's trunk. The root flare should be visible.

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This mulch has been piled too deep and too close to the trunk of the tree. (Submitted photo)
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Q: I am seeing more and more trees in my neighborhoods with mulch piled up on the trunks (photos attached). Can you please explain why mulching like this is not a good idea? And then I’ll send the column to my neighbors!

A: Happy to help.

Mulch around trees is good. Piling the mulch up against the trunk is bad.

Putting wood chips, shredded bark, compost or other organic mulch around a tree can help improve the soil, preserve moisture and protect roots against fluctuations in temperature. Having mulch rather than grass around a tree also can help prevent injuries to the tree from mowers or weed whippers. Trees do better when they don’t have to compete with grass. And grass doesn’t usually grow well under the shade of a tree, anyway.

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Mulching around a tree is one of the best things you can do to preserve its health.

We recommend using organic mulch. Organic doesn’t mean the same thing in landscaping as it means in the grocery store; we just mean mulches that come from plants as opposed to inorganic mulches such as gravel or decorative rocks. Rock mulches can compact the soil, and it’s hard to pull the weeds that pop up among the rocks. Organic mulches such as leaves, bark, pine needles, cocoa shells and wood chips will eventually break down and need to be replaced. But that’s a good thing. As they break down, they improve the structure of your soil.

The mulch should be spread in a layer that is 2 to 4 inches thick. Thicker is not better. If mulch is piled too deep, it can smother the roots.

And, as you say, mulch should not be piled against the trunk. The root flare should be visible. This is where the trunk widens, where the tree meets the ground.

Piling mulch against the trunk can promote disease and decay. It holds in moisture, which can cause the bark to rot. Excess mulch may even kill the phloem, the inner layer that carries nutrients between the roots and the canopy. It can cause a tree to develop encircling roots and eventually strangle itself. It also provides a home for insects and pathogens and can help them make their way into the tree, where they can cause sickness and death.

You may not see these dire effects right away, but making a mulch volcano around your tree risks shortening its life.

Trees are expensive. Protect your investment by making sure to mulch your tree properly, keeping whatever mulch you are using several inches away from the trunk.

Written by U of M Extension Master Gardeners in St. Louis County. Send questions to features@duluthnews.com.

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